Fix errors and drag&drop in UploadBlobModal component

Let's address the concerns that were discovered from this MR > !62880 (merged). The app/assets/javascripts/repository/components/upload_blob_modal.vue seems to have the following issues:

1. The drag & drop feature doesn't seem to work. But clicking the upload does.

drag-drop-file-not-working

2. There is an Uncaught TypeError once a file is uploaded.

Replace file	Upload file